Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for determining a profile of a streamer to be used in a marine seismic survey.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas deposits, it suggests, to those trained in the field, the presence or absence of such deposits. Thus, providing a high-quality image of the subsurface is an ongoing process for the exploration of natural resources.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of acoustic detectors 12, which are distributed along a body 14. The body 14 and its corresponding detectors 12 are sometimes referred to, by those skilled in the art, as a streamer 16. The vessel 10 may tow plural streamers 16 at the same time. The streamers may be positioned horizontally, i.e., lying at a constant depth z1 relative to the ocean surface 18. Also, the plural streamers 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the ocean surface as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. FIG. 2 shows such a configuration in which all detectors 12 are distributed along a slanted straight body 14, making a constant angle α with reference horizontal line 30.
With reference to FIG. 1, the vessel 10 may also tow a sound source 20 configured to generate an acoustic wave 22a. The acoustic wave 22a propagates downward and penetrates the seafloor 24, eventually being reflected by a reflecting structure 26 (reflector R). The reflected acoustic wave 22b propagates upward and may be detected by detector 12. For simplicity, FIG. 1 shows only two paths 22a corresponding to the acoustic wave. However, the acoustic wave emitted by the source 20 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 20. Parts of the reflected acoustic wave 22b (primary) are recorded by the various detectors 12 (the recorded signals are called traces) while parts of the reflected wave 22c pass the detectors 12 and arrive at the water's surface 18. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water's surface acts as a mirror for the acoustic waves), the reflected wave 22c travels back toward the detector 12 as shown by wave 22d in FIG. 1. Wave 22d is normally referred to as a ghost wave because it is caused by a spurious reflection. The ghosts are also recorded by the detector 12, but with a reverse polarity and a time lag relative to the primary wave 22b. The degenerative effect that ghost waves have on bandwidth and resolution of seismic measurements is known. In essence, interference between primary and ghost arrivals causes, among other problems, notches, or gaps, in the frequency content of the data recorded by the detectors.
The traces may be used to determine the subsurface (i.e., earth structure below surface 24) and to determine the position and presence of reflectors 26. However, the ghosts disturb the accuracy of the final image of the subsurface, and for at least this reason, various methods exist for removing the ghosts, i.e., deghosting, from the results of a seismic analysis.
The streamer configuration illustrated in FIG. 2 is considered to provide a clearer image of the subsurface than the configuration illustrated in FIG. 1. One reason for this difference is that for each reflector, the time gap between detection of the primary and ghost reflections becomes greater the further the detector 12 is from the source 20, due to the detectors' slanted disposition, thus facilitating deghosting.
However, the slanted streamer shown in FIG. 2 has the following limitation, which makes it impractical. Current streamers have a typical length of about 6 to 10 km. Using a slanted streamer as suggested in U.S. Pat. No. 4,992,992, e.g., with a slope of 2 percent (i.e., 0.02) relative to the horizontal line 30, would lead to a depth of about 280 m for the last detector, while in reality current marine detectors are designed to operate in water depths up to about 50 m. Thus, for current streamers, the approach proposed in the '992 patent would require detectors to be located in water depths beyond their current capabilities, resulting in detectors' failure or making it impossible to deploy detectors at those depths.
The effect of ghosts on the frequency spectrum is known in the art and has been discussed, for example, in provisional Patent Application No. 61/392,982, entitled, “Method and Device to Acquire Seismic Data,” authored by R. Soubaras, the same inventor as this patent application.
Thus, it is desirable to perform data acquisition using a broad bandwidth of frequencies to reduce ghosts. The broad bandwidth is understood to include low frequencies (e.g., 0 to 32 Hz) and high frequencies (e.g., 68 to 132 Hz). Broad bandwidth is desirable because it produces sharper wavelets for better resolution of important features such as thin beds and stratigraphic traps. Low frequencies provide better penetration for deep targets, as well as better stability for inversion.
At least two octaves of signal are required for seismic imaging, and more are better. An effect of increasing the low-frequency content of collected seismic data is to decrease the side lobes of the wavelet, thus making a more accurate interpretation. Increasing the high-frequency content sharpens the central peak of the wavelet, yet still leaves reverberating side lobes, making precise interpretation difficult. The sharpest wavelets, and therefore the best resolution, are produced by extending the bandwidth in both the low- and high-frequency directions. It is possible now to record a full six octaves of signal by using BroadSeis equipment and algorithms (as developed by CGGVeritas, Massy, France). The BroadSeis equipment includes curved streamers and/or other variable-depth streamers as disclosed in patent application Ser. No. 13/272,428, “Method and Device to Acquire Seismic Data,” authored by R. Soubaras, the entire content of which is included herein by reference.
As can be seen from the above discussion, there is a need to provide a method for performing a marine seismic survey in which the contribution of the ghost may be eliminated or separated from the contribution of the primary to improve the quality of a subsurface image. The BroadSeis method uses a variable-depth streamer; however, the streamer's shape may vary with the purpose of the seismic survey and the structure of the subsurface to be surveyed. Accordingly, it would be desirable to provide systems and methods that determine the shape of the streamer for a given subsurface.